Generally linear digital hydraulic actuators are limited in those cases where a double action arrangement is required as the total area or range of pressures that may be applied are limited since the forces must be applied to move in both directions. This limits the variation in pressure that may be applied in any one direction or increases significantly the size of the actuator, see U.S. Pat. No. 4,602,481 issued July 29, 1986 to Robinson which discloses a linear double acting digital system of the kind described.
U.S. Pat. No. 3,068,841 issued Dec. 18, 1962 to Robbins et al discloses a system to permit rapid advance of a ram toward a workpiece under low power and low pumping volume requirements and for effecting full force against the work piece after the ram is positioned by utilizing different piston and cylinder sizes to obtain the desired results.
Generally digital actuators are separate elements and are connected to discrete valves and separate reservoir systems all arranged in positions remote from the actuator itself.